X-ray imaging is commonly used as a diagnostic tool in dental settings. Traditionally, film was used as the x-ray detector. The film is inserted into the patient's mouth behind the teeth and the outside of the patient's jaw is exposed to a pulse of x-rays. The teeth absorb the x-rays, and hence, create contact images of the teeth on the film which are then used by the dentist to diagnose the state of the teeth.
This system had a number of problems that have been overcome to some degree by replacing the film with a digital image sensor. In such systems, a scintillator is used to convert the x-rays to visible light. The light is collimated by a fiber optic bundle, and a CMOS image sensor of substantially the same type as used in digital photography is used to detect the generated light that leaves the collimator.
The time and cost of processing the film are substantial in a film-based system. Converting the film images to digital form for storage in a modern record keeping system further increases the cost of such systems both in terms of equipment and operator time. Digital image sensors avoid the film processing and provide a final image that is in a form compatible with digital record storage systems.
Determining the correct exposure presents problems with film which are partially solved by the use of digital sensors. The dynamic range of the film is typically less than a factor of 100. That is, the ratio of the highest light level that can be recorded without saturating the film to the lowest level that can be detected is of the order of 100. This dynamic range is sufficient to provide useable images if the exposure is correctly set. However, setting the exposure presents challenges. If too short of an x-ray exposure is used, the image will lack detail in the darker areas. If the x-ray exposure is too high, part of the image will be overexposed and insufficient detail will be present. The correct exposure, in general, depends to some degree on the structure of the patient's face, as the patient's face absorbs part of the x-ray before the x-rays reach the teeth.
Digital sensors have substantially higher dynamic ranges than film. Hence, even if the exposure is above or below the optimum level needed to place the image in the linear range of the sensor response function, a useable image can be obtained by digitally processing the image. However, providing an optimal exposure is still advantageous.
Digital sensors also require substantially less x-ray exposure to form a usable image. In general, it is advantageous to reduce the x-ray exposure to which the patient is subjected to as low a level as possible. Hence, digital sensors hold the promise of reducing this exposure. However, this promise has only partially been achieved by current digital x-ray systems.
In general, the x-ray generating system is independent from the imaging sensor. For example, a dentist who desires to switch from film to a digital sensor, typically uses the dentist's existing x-ray system and merely reduces the exposure to a level that provides satisfactory images. The technician places the digital sensor in the patient's mouth and then triggers the x-ray system to provide a pulse of x-rays that is sufficient to provide the desired image. The digital sensor sends the image to a computer that processes the image and stores the data in the electronic patient information system.
Systems in which the digital sensor detects the start of the x-ray pulse and begins its exposure when the pulse is detected have been suggested. Such systems can also end the exposure when sufficient x-rays have been detected by the same sensor mechanism. However, these systems do not have any mechanism for turning off the x-ray generator when there is sufficient data to form an image, and hence, the patient is subjected to additional x-ray exposure that provides no therapeutic benefit.
Schemes in which a radiation detector associated with the imaging array measures the x-ray dose during the exposure and then turns off the high-voltage supply to the x-ray generator have also been suggested. These systems require an x-ray generator in which the high-voltage can be controlled by the electronics that monitor the exposure measuring device and/or the imaging system. However, most dental offices utilize x-ray systems that were designed for film-based x-ray imaging and do not provide the remote control system required by such automatic exposure systems. In these legacy systems, the operator initiates an exposure of a predetermined length by pressing a button. To protect the operator from repeated exposures, the button is placed at a remote location relative to the patient and x-ray head. In addition, the electronics that control the x-ray exposure do not present a readily accessible location that can be used to fashion a remote control interface that can be used by automatic exposure control systems.